Evaporator defrosting means



Sept. 22, 1959 PROS T FORMAT/0N .J. H. SPENCER, JR EVAPORATOR DEFROSTINGmus Filed Nov. 1, 1956 PEA K RESONANCE PULL IN v DROP oui' I INVENTOR.

JEss/E H'SPENCERJR ATTORNEY United States Patent 2,904,968 EVAPORATORDEFROSTING MEANS Jessie H. Spencer, Jr., Germantown, Ohio, assignor toGeneral Motors Corporation, Detroit, Mich., a corporation of DelawareApplication November 1, 1956, Serial No. 619,798 5 Claims. (Cl. 62-140)This invention relates to refrigerating apparatus and more particularlyto automatic defrosting effected from time to time as the frost buildsup.

The common procedure is to defrost the refrigerant evaporator manuallyor by some form of timer to defrost the evaporators at regular periodicintervals. Since the frost accumulation varies in proportion to the timethe door is open and to the humidity in the room at the time the door isopen, the timing device must be set to defrost the evaporator atintervals sufiiciently long and frequent with sufiicient heat developedduring the defrosting period to accomplish the defrosting under theworst frosting conditions. While attempts have been made to initiate andcontrol defrosting cycles by the thickness of the frost, as far as I amaware, none has proved practical.

It is an object of this invention to provide a practical method of andapparatus for defrosting refrigerant evaporators whenever there is apredetermined accumulation of frost and for continuing the defrostinguntil the frost has substantially disappeared.

It is another object of this invention to provide a defrosting system inwhich the accumulation of frost causes a change in a resonant circuit toinitiate a defrosting period and the cessation of refrigeration and inwhich the return to normal of the resonant circuit causes thetermination of the defrost period and the resumption of refrigeration.

These and other objects are obtained in the form of the invention shownin the drawings in which a metal plate is positioned close to andparallel to a vertical surface of the evaporator but insulated therefromso as to form a capacitor with the adjacent metal wall surface thereof.The metal plate and the evaporator are connected as a capacitor in aresonant electric circuit including a relay. The circuit is arranged sothat under normal conditions when there is no frost between evaporatorand the metal plate, substantially no current flows through the relaycoil and the relay contacts connect the refrigeration system to thepower supply. Whcnsufiicient frost accumulates between the evaporatorand the plate, the dielectric constant is increased, the current flow inthe circuit increases as the circuit approaches resonance, causing therelay to operate to disconnect the refrigerating system from the powersupply and to connect an electric heat defrosting system to the powersupply. Under such circumstances the electric heat quickly defrosts theevaporator. The defrost water can readily flow away from the spacebetween the conductor metal plate and the evaporator. The dielectricconstant is thereby restored to the original condition at the completionof the defrosting and the resonant circuit is returned to the state inwhich substantially no current flows through the relay. The relay willthen return to its normal position in which the defrost heater isdeenergized and the refrigerating system is reenergized to resumerefrigeration.

Further objects and advantages of the present invention will be apparentfrom the following description, refrator 20. The amount of frostdeposited varies with the. length and frequency of the open periods ofthe door ICC of Figure 1 showing the frost upon the evaporator surfaces.

Referring now to the drawings there is shown arefrigerant evaporator 20provided with hollow metal walls within which the liquid refrigerantevaporates and is drawn through the return conduit 22 by the sealedmotor compressor unit 24. This sealed unit 24 compresses the refrigerantand forwards it to the condenser 26 where it is liquefied and flowsunder the control of a suitable valve or restrictor 28 back to thehollow walls of the evaporator 20. The sealed unit 24 may be controlledby any suitable form of thermostatic switch 30 responsive tothetemperature of the evaporator 20 or the medium cooled thereby. Astarting relay 32 may also be pro vided. The evaporator 20 may belocated within an insulated household refrigerator cabinet 34 providedwith an access door.

Since the evaporator 20 is used for the freezing of the.

ice as Well as for cooling the air within the insulated cabinet 34, theevaporator 20 must operate at freezing temperatures. Whenever air iscooled at freezing temperatures below the dew point, some of themoisture. in the air will be deposited as frost upon the freezingsurfaces. When the access door is open, the Warm air from the roomenters the cabinet and moisture from the air is deposited as frost uponthe surfaces of the evapoand with the humidity of the air which entersthe door.

According to my invention whenever a predetermined amount of frostaccumulates on the outer surface of the evaporator 20, refrigeration isdiscontinued and a defrosting period is begun which continues until thefrost is substantially all melted from the evaporator. This is accomplished by a high impedance resonant circuit which includes a doublethrow switch 36 having a spring blade with double contacts at one endwhich is normally sprung so as to make contact with the upper contact38. This upper contact 38 connects through the conductor 40, the.

thermostatic switch 30, the conductor 42 and the starting relay with thesealed unit 24. The starting relay 32 and the sealed unit 24 are alsoconnected by the conductor 44 to the supply conductor 46. The doublethrow switch 36 is connected by the'conductor 48 to the second supplyconductor 50. The double throw switch 36 is provided with an armature 52cooperating with the electromagnet 5 4 1 to form a relay. Thiselectromagnet 54 has one terminal connected by a conductor 56 with athin metal plate 58 spaced from and insulated from the adjacent" wallsof the evaporator so that the two metal surfaces become a capacitor withthe air, water, or frost between them serving as the dielectric. Therefrigerating system, particularly the evaporator, is grounded to themetal parts of the cabinet as indicated by the reference character 60.

Connected between the supply conductors S0 and 46 is the primary coil 62of a transformer 64. The transformer 64 has a secondary winding 66supplying a low voltage such as 30 volts, and having one terminalconnected by the conductor 68 to the ground 70. The other terminal ofthe secondary winding 66 is connected by the conductor 72, the coil 74and the conductor 76 to the second terminal of the electromagnet 54.Connected between the conductors 68 and 76 is a capacitor 78. This highimpedance circuit is so designed that the combined impedance of thecoils 74 and 54 equals the combined impedance of the capacitors 78 and58-20 when frost accumulates to an undesirable extent upon the surfaceof the evaporator 20 and particularly between the plate 58 and theadjacent metal wall surface of the evaporator 2%), the circuitappreaches resonance. This circuit includes the secondary winding 66,the conductor 72, the coil 74, the conductor 76, the electromagnet coil54, the conductor 56 and plate 58, the evaporator 20, the grounds 60 and70, the conductor 68 and the capacitor 78.

When the circuit approaches resonance, sufficient current flows throughthe electromagnet coil 54 to attract the armature 52 and pull the doublethrow switch 36 downwardly into engagement with its lower contact 80.The lower contact 80 connects to this one terminal of the heaters 82which surround the outer surface of the evaporator 20 in such a way thatthe frost will be heated to the point of melting. The electric heaters82 have a second terminal connected by the conductor 84 to the supplyconductor 46. This operation causes a cessation of the Operation of therefrigerant liquefying means which includes the unit and the condenser26 and the valve 28 and causes the generation of heat by the heaters 82until the frost substantially all melts from the evaporator 20. Themelting of the frost between the metal plate 58 and the adjacent.metalwall of the evaporator 20 provides decreased dielectric constant betweenthe plate 58 and the metal wall of the evaporator 28 to restore itscapacitance to its normal value. This throws the circuit out ofresonance and restores the high impedance thereof which re-.

duces the current flow through the electromagnet 54 to a minimum asshown in Figure 2. This weakens the attraction for the armature 52 andcauses the spring force of the double throw switch 36 to move away fromthe lower contact 80 back to engagement with the upper contact 33 torestore the refrigerating system to normal operation and to deenergizethe electric heaters 82.

l'Similar defrosting cycles will occur whenever sufficient frostaccumulates between the metal plate 58 and the adjacent metal wall ofthe evaporator 20. In this way defrosting as much and no more thannecessary is provided automatically. The circuit and the arrangement aresimple and inexpensive.

While the form of embodiment of the invention as herein disclosed.constitutes a preferred form, it is to be understood that other formsmight be adopted, as may come within the scope of the claims whichfollow.

What is claimed is as follows:

i .1. In refrigerating apparatus including refrigerant evaporating meanshaving an electrically conductive surface and refrigerant liquefyingmeans operatively connected to said evaporating means, means associatingan electrical conductor plate in spaced relation to said conductivesurface of said evaporating means to form capacitor means, andelectrical means responsive to a change in capacitance of said capacitormeans for initiating a defrosting cycle of said evaporating means.

2. In refrigerating apparatus including refrigerant evaporating meanshaving an electrically conductive surface and refrigerant liquefyingmeans operatively connected to said evaporating means, a resonantcircuit including said conductive surface and an electrical conductorplate associated in spaced relation to said conductive surface, andmeans responsive to a change in resonance of said resonant circuit forinitiating a defrosting cycle of said evaporating means.

3. In refrigerating apparatus including refrigerant evaporatingmeanshaving an electrically conductive surface and refrigerant liquefyingmeans operatively connected to said evaporating means, means associatingan electrical conductor plate in spaced relation to said conductivesurface of said evaporating means to form capacitor means, a capacitanceconnected in parallel electric circuit arrangement with said conductivesurface and said conductive plate, a power supply having supplyconductors connected to said circuit arrangement on opposite sides ofsaid caacitance, a choke coil connected in series with one of saidsupply conductors, a relay having its operating element connected inseries with said parallel circuit arrangement, said relay having meansfor initiating aldefrosting cycle.

4. In refrigerating apparatus including refrigerant evaporating meanshaving an electrically conductive surface and refrigerant liquefyingmeans operatively connected to said evaporating means, means associatingan electrical conductor plate in spaced relation to said conductivesurface of said evaporating means to form capacitor means, a capacitanceconnected-in parallel electric circuit arrangement with said conductivesurface and said conductive plate, a power supply having supplyconductors connected to said circuit arrangement on opposite sides ofsaid capacitance, a choke coil connected in series with one of saidsupply conductors, a relay having its operating element connected inseries with said parallel cir' cuit arrangement, electrically operateddefrosting means forsaid evaporating means, said relay having onealternate position for operating said liquefying means and for renderinginoperative said defrosting means and a second alternate position foroperating said defrosting means and rendering inoperative .saidliquefying means.

5. In refrigerating apparatus including refrigerant evaporating meanshaving an upright electrically conductive surface exposed to air to becooled and refrigerant lique' fying means operatively connected to saidevaporating means, a resonant circuit including said conductive surfaceand an upright electrical conductor plate associated in spaced insulatedrelation to said conductive surface in the air to be cooled to form acapacitor in which the capacitance will change according to the amountand distribution of frost between said plate and said conductivesurface, and means responsive to a change in resonance of said resonantcircuit for initiating a defrosting cycle of said evaporating means.

References Cited in the file of this patent V UNITED STATES PATENTSOhlheiser Sept. 13, 19

